An increasing number of sensors are integrated into semiconductor chips. Such a sensor chip may be placed on a carrier such as a die pad, and possibly be encapsulated.
In some applications, the sensor chip is thinned from a back side of the sensor chip for building a thin membrane that is thermally isolated versus the rest of the sensor chip. This may serve the operation of a sensitive element on or in the membrane, for example, in particular when the sensitive element is provided for sensing a temperature or otherwise is sensitive to temperature, or, for example, when a heating operation is required during measuring or manufacturing which heat is not desired to migrate elsewhere to the chip but is desired to remain locally confined, e.g. in the membrane.
When such a sensor chip is mounted to a die pad with its back side facing the die pad, a cavity is generated between the sensor chip and the die pad in view of the recess in the sensor chip. In such arrangement, the membrane of the sensor chip lowers the total mechanical resistance of the sensor package, and the underlying cavity may promote an accumulation of moisture, for example, during manufacturing, shipment or handling. In particular, moisture may be absorbed by an encapsulation of the sensor chip containing organic material, e.g. plastics, if applicable, which moisture may be released into the cavity in response to elevated temperatures. During assembly, for example, a solder reflow process may induce elevated temperatures to the entire sensor package including the encapsulation if any. However, elevated temperatures may also be induced from external, or from internal, wherein during operation, for example, a heater may induce elevated temperatures if applicable. Irrespective of the source of heating, such elevated temperature may result in a vapor pressure increase inside the cavity, and in a plastic encapsulation of the sensor chip if applicable. Such moisture uptake may finally lead to delamination of material interfaces and/or to cracks in the sensor package which is also referred to as “popcorn” phenomenon.
For moisture/reflow sensitive components such as plastic-encapsulated surface mounted devices and other packages made with moisture-permeable materials, a moisture sensitivity level is determined by tests and a resulting classification determines storage conditions, packing, handling and timing specifications for the product at the manufacturer and the customer. In case the sensitivity of a sensor package to moisture can be reduced and less stringent moisture sensitivity levels can be achieved, packing requirements may be reduced and the handling of the sensor package may be facilitated. However, drilling a hole in the die pad underneath the cavity does not seem to solve the problem given that such hole provides access to the cavity and the membrane in the following which may allow dust, flux, PCB coating material, assembly chemicals, etc. enter the cavity e.g. during assembly of the sensor package itself, e.g. during dicing, handling, testing, etc., during shipment, during assembly on a PCB at costumer side, and during whole operating lifetime.
Hence, it is desired to provide a sensor package that is less prone to a vapor pressure increase in the cavity.